Method of producing crystalline boric oxide



Patented Nov. 15, 1938 UNITED STATES PATENT OFFICE METHOD OF PRODUCINGCRYSTALLINE BORIC OXIDE Leon McCulloch, Pittsburgh, Pa., assignor toWestinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., acorporation of Pennsylvania No Drawing. Application July 31, 1937,,Serial No. 156,829

5 Claims.

having a moderate purity which can be purchased in the open market.

These technical grades of the boric acid are to be preferred for theformation of the first seed crystals since it has been found throughexperiments that the growing of the crystals may be accomplished at afaster rate with the moderately pure boric acid than where chemicallypure boric acid is employed.

In order to induce an initial crystallization, the crystals of which maybe employed for the purpose of seeding other liquids, as describedhereinafter, a quantity of the moderately pure boric acid may be placedin a suitable container, not shown, which is open to the atmospherethrough a small vent and then subjected to a substantially constanttemperature above the melting point of boric acid to fuse it until itbecomes a viscous liquid. The heating of the boric acid in'the containermay be by any suitable means, although it is preferred that the heatingbe accomplished in an electric furnace provided with an automatictemperature control for controlling the heating temperature.

The heating may be continued until all of the excess steam escapes tothe atmosphere and an examination of the viscous liquid reveals that thefused boric acid retains water in solution in an amount ranging from 8%to 14% by weight. It has been found necessary to have water in solutionin the fused boric acid within the ranges given in order thatcrystallization of the liquid may be obtained. This is the percentage ofwater that remains in boric acid when fused at atmospheric pressurewithin the temperature range employed. If higher temperatures wereemployed, it would be necessary also to go to pressures higher thanatmospheric pressure.

With the content of the water in solution ranging from 8% to 14% byWeight and preferably from 10% to 12% by weight at atmospheric pressure,the viscous liquid may be further subjected to the substantiallyconstant temperature. When subjected to the substantially constanttemperature for a period of time of from 1 to 30 days, it has been foundthat minute crystalline bodies begin to develop and form about the wallsof the container. An examination of the minute crystalline bodiesdeveloped in the viscous liquid reveals that they are of rounded orspheroidal shape of about .3 millimeter in diameter. Continuedapplication of the substantially constant temperature to the viscousliquid causes the initial minute crystalline bodies to multiply with theliberation of water vapor until the whole of the liquid in the containerbecomes solidified. If the viscous liquid is examined when only 1%remains uncrystallized,

it will be found that the 1% of the liquid still has a content of waterin solution in the boric acid ranging from 8% to 14% by weightindieating that this content is necessary in order that thecrystallization may be completed. As an example of the speed at whichthe viscous liquid crystallizes, a liter of the boric acid of moderatepurity completely crystallized in a period of 7 days when heat treatedas described hereinbefore whereas a liter of boric acid of chemicalpurity required twice as long a time.

In heating the viscous liquid to induce the initial crystallization andcause the liquid to completely crystallize, it is necessary that thetemperature of the heating be below the temperature at which thecrystals will dissolve and also to maintain the content of waterin solution with the fused boric acid within the ranges given hereinbefore. Aparticularly effective heating range has been found to be between 220 C.and 250 'C.

In order to produce the crystalline boric oxide in quantity, thecrystals formed by the initial crystallization described hereinbeforemay be employed as seeds for initiating the crystallization of otherviscous liquids of fused boric acid. The initial crystals from theviscous liquid in the container before the liquid has become completelycrystallized, or the solidified initial crys talline mass may bepowdered and a portion thereof may be employed as seeds to inducecrystallization of other viscous liquids of boric acid.

In producing the crystalline boric oxide in quantities, a quantity ofboric acid may be fused in a similar manner to that described inproducing the initial crystallization. After being fused underatmospheric pressure at a temperature above the melting point of boricacid and brought to a temperature of between 225 C. and

o/ll

250 C., the seeds obtained from the initial crystallization may beintroduced into the viscous liquid.

The crystalline boric oxide seeds added to the viscous liquid may be inany desired amount, although it is found that crystalline boric oxideseeds of the order of .l% to 1% by weight of the liquid to becrystallized are sufficient to cause a rapid crystallization of theviscous liquid. If a larger quantity of the initial crystals or seeds ofboric oxide is added to the viscous liquid the crystallization actionwill be carried to completion in a shorter period of time.

An examination of the viscous liquid during 5 the crystallizationtransformation reveals that at first a cloudiness is formed in the clearfused boric acid which increases with the escape of steam as bubblestherefrom, until the liquid becomes more and more pasty and finallysolid. The crystals obtained are of microscopic size, the resultingcrystalline mass being an anhydrous, white, opaque, non-glasslike, stonysolid and strong product which is somewhat resilient under blows from ahammer.

Since the crystalline boric oxide is free of water and, therefore,non-volatile, it is particularly useful in certain branches ofindustrial chemistry and in particular glass making. Other uses of thecrystalline boric oxide produced by this invention are as reagents inanalytical chemistry or as fluxes for use in welding as in metallurgy. Acrystalline boric oxide produced by this invention may also be employedin the production of elemental boron and boron alloys by reduction withmagnesium or other active metals. A further use of the crystalline boricoxide of this invention is as a desiccating agent, since it can take upabout 77% of its weight of water in going back to boric acid.

Although this invention has been described with reference to aparticular method, it is, of course, not to be limited thereto exceptinsofar as is necessitated by the prior art and the scope of theappended claims.

I claim as my invention;

1. The method of producing crystalline boric oxide which comprises,fusing boric acid and subjecting it to a temperature above its meltingpoint to form a viscous liquid, continuing the heating to drive offmoisture from the liquid until the content of water in solution in theliquid is sufficient to cause crystallization of the viscous liquid,continuing the heating of the liquid containing the water in solution tocause crystalline v) bodies of boric oxide to be formed with aliberation of water vapor, the heating temperature being maintained at avalue below the temperature at which the crystals dissolve until adesired amount of the crystalline boric oxide is developed, the contentof the water in solution in the uncrystallized portion of the liquidmaintaining a substantially contant ratio asthe crystals are formed.

2. The method of producing crystalline boric oxide which comprises,fusing boric acid and subjecting it to a temperature above its meltingpoint to form a viscous liquid, continuing the heating to drive offmoisture from the liquid until the content of the water in solution atatmospheric pressure is between about 8% and 14%, and continuing theheating of the liquid containing the water in solution to causecrystalline bodies of boric oxide to be formed with a liberation ofwater vapor, the heating temperature being maintained at a substantiallyconstant value below the temperature at which the crys: tals dissolveuntil the desired amount of the crystalline boric oxide is developed.

3. The method of producing crystalline boric oxide which comprises,fusing boric acid and subjecting it to a temperature above its meltingpoint to form a viscous liquid, seeding the viscous liquid with a minorportion of crystalline boric oxide, and continuing the heating of theseeded viscous liquid to cause the crystallization of the liquid toproceed with a liberation of water vapor therefrom, the heating of theseeded viscous liquid being at a temperature above the melting point ofthe boric acid and below the temperature at which the seed ofcrystalline boric oxide dissolves.

4. The method of making crystalline boric oxide which comprises, fusingboric acid and sub- 1 jecting it to a temperature above its meltingpoint to form a viscous liquid, seeding the viscous liquid with a smallbut effective amount up to 1% by weight of crystalline boric oxide, and

continuing the heating of the seeded viscous liq- :n"

above its melting point to .form a viscous liquid,

continuing the heating to drive 01f moisture from the liquid until thecontent of the water in solution at atmospheric pressure is betweenabout 8% and 14%, continuing the heating of the viscous liquidcontaining the water in solution to cause crystalline bodies of boricoxide to be formed, removing a portion of the crystalline bodies fromthe container and introducing them into a second container having aviscous liquid of fused boric acid therein, the viscous liquid in thesecond container being heated at a temperature of betweenthe meltingpoint of boric acid and the temperature at which the crystalline boricoxide dissolves, and continuing the heating of the liquid in the secondcontainer to cause the crystallization of the viscous liquid to proceedwith the evolution of water vapor therefrom.

LEON MCCULLOCH.

